Conventionally, such engine is already known from Japanese Patent Publication No. 60-10165 and the like, for example.
In the conventional SOHC-type engine, a pair of rocker shafts are disposed on opposite sides of and above the camshaft, and an intake-side rocker arm moved following an intake-side cam on the camshaft to drive the intake valves is swingably carried on one of the rocker shafts, and an exhaust-side rocker arm moved following an exhaust-side cam on the camshaft to drive the exhaust valves is swingably carried on the other rocker shaft. Therefore, the width of the cylinder head in a direction perpendicular to the camshaft is obliged to become relatively large.